This invention relates in general to radio receivers and more particularly to the avoidance of lost data and information due to long receiver recovery times due to demodulator transients.
In some instances, a radio frequency (RF) receiver may, by virtue of its design, have signal alternating current (AC) coupling circuitry where a highly coupled signal using a substantially high resistor-capacitor (RC) time constant will have a slow xe2x80x9cattack timexe2x80x9d in view of this signal coupling. That is, since the signal coupling circuitry is generally used to block direct current (DC) while still passing very low frequencies, some data or information will be misprocessed when first received. More specifically, the first syllable of a speech message or the initial bit(s) of a data stream will be lost since a center slicer (discussed hereinafter) will not properly convert data from the demodulator immediately following a signal transient.
As is well known in the art, a signal transient is any non-periodic perturbation in the received signal. There are any number of causes of signal transients which result in the loss of information. One of these is commonly referred to as xe2x80x9cnetting errorxe2x80x9d. Netting error occurs where the transmitted frequency and received frequency are not identical. This tends to cause a transient in the form of a DC shift at the output of the receiver demodulator. This in-turn causes errors in decoding data such as private line (PL), digital private line (DPL), digital tone multi-frequency (DTMF), etc. Where digital information from the receiver demodulator is converted by the center slicer or analog-to-digital converter (ADC) which misrepresents the original data.
Thus, it is desirable to know when a demodulator transient occurs so that it can be compensated for such that an ADC can act as a true center slicer to the extent that information can be correctly interpreted by a decoder and/or microprocessor as quickly as possible following the transient. If the transient is not detected and compensated for quickly, the center slicer will take a greater amount of time to output valid data. During that time period when the DC offset is not detected and compensated for, the center slicer generally outputs a series of constant 1""s or 0""s. A decoder or microprocessor then attempts to decode this erroneous information which cannot be done. This ultimately means that the radio receiver will be muted during this time period that erroneous information is sent to the decoder and any incoming voice will be lost until that DC offset can again properly compensated for.
The above system is illustrated in prior art FIG. 1 where a typical receiver circuit 100 includes a receiver detector 101, and demodulator 103. As seen in FIG. 2A, when an RF signal is first received and netting error is present, the output of the demodulator is DC shifted where the RF signal rides upon the DC shifted signal by some predetermined amount. This signal is then typically coupled i.e. DC blocked using an resister-capacitor time constant formed using capacitive coupling 105 and a load 107. This coupled signal is then input to a filtering network 109. As seen in FIG. 2B, upon the occurrence of the DC shift, the input signal is initially shifted up to the predetermined DC offset where its DC offset then decays exponentially at a rate depending on the specific resister-capacitor (RC) time constant selected. FIG. 2C depicts the output of the center slicer 111 before being input to the decoder 113 where upon occurrence of the DC shift, the input of the decoder is input with a continuous series of 1""s. Since this erroneous information cannot be decoded, information is lost until the DC offset shown in FIG. 2B decays close enough to xe2x80x9ccenterxe2x80x9d that the center slicer output again represents the data accurately enough to be decoded.
One method currently used to mitigate this problem is to speed up the RC time constant for a predetermined period of time through the use of software in the receiver. Although some incoming information is still unavoidably lost due to the DC shift and also because the AC portion of the signal becomes highly attenuated via the smaller RC time constant, this method allows the receiver to recover at a much quicker rate. This method is commonly referred to as an xe2x80x9cadapt functionxe2x80x9d which allows the receiver to speed up its attack time. If the user knows that this is going to be an issue and information may be lost, for example in a xe2x80x9cscan modexe2x80x9d where one knows that netting error will occur when the receiver xe2x80x9clandsxe2x80x9d on the channel if a carrier exists, the adapt function can be initiated. This ultimately allows the receiver to attack and recover an incoming signal at a higher rate such that incoming information lost due to a demodulator transient is minimized. However, this method only works when software has apriori knowledge that information will be lost and is not self-adaptive. In other words, it is not a solution to the most common situation where the receiver is operated idle, or muted, until information is received, and software does not know when to change the mode of the receiver to alter its time constant.
Thus, the need exists to provide a circuit that can automatically adapt to receiver demodulator transients such as a DC shift due to netting error, in order to prevent the loss of incoming information or data.